Automobiles are propelled by rotating tires with the rotational drive power generated by an engine such as an internal combustion engine, a motor, or the like and transmitted from a differential gear through a plurality of transmission shafts such as a half shaft, a spline shaft, etc. to hubs. The transmission shafts are connected to each other by a constant velocity universal joint.
A joint structure 1 shown in FIG. 19 includes a Birfield constant velocity universal joint 3 by which a spline shaft 2 is connected to a hub, not shown. The Birfield constant velocity universal joint 3 includes a hollow outer member 4 having an open end, and a joint boot 5 is mounted on and extends between the outer member 4 and the end of the spline shaft 2. The joint boot 5 functions as a seal for holding a grease composition which fills the joint boot 5 and preventing external foreign matter such as water, mud, etc. from finding its way into the joint boot 5 (see, for example, Patent Document 1).
Since the outside diameter of outer member 4 is larger than the diameter of the spline shaft 2, the joint boot 5 comprises, as shown in FIG. 19, a large-diameter tube 6 in which the outer circumferential wall of the outer member 4 is inserted, a small-diameter tube 7 in which the spline shaft 2 is inserted, and a bellows 8 interposed between the large-diameter tube 6 and the small-diameter tube 7, the bellows 8 being progressively smaller in diameter from the large-diameter tube 6 to the small-diameter tube 7.
The joint boot 5 is made of rubber or resin and is flexible. When the automobile runs on a bumpy road, the spline shaft 2 is displaced so as to be tilted with respect to the outer member 4 of the Birfield constant velocity universal joint 3, causing the bellows 8 to flex to follow the displacement. As a result, the bellows 8 is brought into a state shown in FIG. 20.
As can be seen from FIG. 20, as the bellows 8 flexes, the inner circumferential wall of the small-diameter tube 7 near the bellows 8 is spaced from the spline shaft 2, causing a so-called peeled-off phenomenon. When such a phenomenon occurs, the grease composition filling the joint boot 5 may leak out. For this reason, as shown in FIGS. 19 and 21, it has been customary to tighten the small-diameter tube 7 around the circumferential side wall of the spline shaft 2 with a metal band 9 as a tightening member. The large-diameter tube 6 is also tightened around the outer circumferential wall of the outer member 4 by a metal band 10.
When the automobile is steered, the spline shaft 2 is displaced so as to tilt with respect to the outer member 4 of the Birfield constant velocity universal joint 3, and the bellows 8 flexes to follow the displacement.
At this time, as shown in FIG. 22, the large-diameter tube 6 tends to abut against the metal band 10 and the region of the bellows 8 which is closest to the large-diameter tube 6 tends to abut against the end face of the outer member 4. When a metal member such as the metal band 10 or the outer member 4 repeatedly abuts against the joint boot 5 which is made of rubber of resin, the joint boot 5 may possibly be damaged.
In order to avoid the above shortcoming, Patent Document 2 has proposed that the outside diameter of a valley disposed adjacent to the large-diameter tube be in the range from 80% to 90% of the outside diameter of the large-diameter tube, and that the wall thickness of a slanted portion of the valley which is closer to the large-diameter tube be greater than the wall thickness of other slanted portions of the bellows.
The small-diameter tube 7 may also possibly be damaged. Specifically, even though the small-diameter tube 7 is tightened by the metal band 9, it is difficult to fully prevent the small-diameter tube 7 from being peeled off, and the small-diameter tube 7 tends to be slightly peeled off. When the small-diameter tube 7 is peeled off, the metal band 9 abuts against a portion of the joint boot 5, causing that portion to subside. Therefore, if the small-diameter tube 7 is repeatedly peeled off, then the portion of the joint boot 5 which abuts against the metal band 9 may be damaged.
For the purpose of preventing the portion which abuts against the metal band from being damaged, Patent Document 3 has proposed that stiffening fins be provided on the outer surface of the bellows which is close to the small-diameter tube. Patent Document 4 has proposed that a joint boot have a recess for distributing the distortions.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-263730;
Patent Document 2: Japanese Laid-Open Patent Publication No. 09-014283;
Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-099331; and
Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-301202.